This invention relates generally to optimization of digital communication networks. More particularly, it relates to using forward error correction for optimization of variable parameters of digital fiber optic communication networks.
FIG. 1A is a simplified schematic diagram of a conventional single-channel digital communication network 100. A digitized signal 101 is launched from a transmitter 102 through a transmission medium 104, and is received and processed at a receiver 106. Typically, transmission medium 104 is an electrically conducting cable or an optical fiber.
FIG. 1B is a simplified schematic diagram of a conventional multi-channel digital communication network 120. In multi-channel network multiple signals 121a, . . . , 121n are launched from respective multiple transmitters 122a, . . . ,122n in a transmitter array 132 through a transmission medium 134 having multiple channels 124a, . . . , 124n and are received and processed at respective multiple receivers 126a, . . . , 126n in a receiver array 136. Transmission medium 134 can be an electrically conducting cable or an optical fiber. Multiple signals 121a, . . . , 121n are commonly multiplexed. Particularly, in an optical digital network the multiple signals are commonly multiplexed using a wavelength division multiplex (WDM) technique. Further, in a multiplexed digital network, the multiplexed signals 121a, . . . , 121n are commonly amplified between transmitter array 132 and receiver array 136 using a common amplifier 138.
Forward Error Correction (FEC) has been adopted as a standard technique for improving the performance of digital optical communication networks [International Telecommunication Union (ITU) G.975 xe2x80x9cForward error correction for submarine systemsxe2x80x9d ]. The FEC function comprises a FEC encoder in the transmitter that accepts information bits and adds computed redundant symbols, producing encoded data at a higher bit rate; and a FEC decoder in the receiver that performs the error correction while extracting the redundancy to regenerate the original data.
The FEC function allows in-line monitoring of the line bit-error ratio (BER) before correction through knowledge of the exact number of corrected bits and keeps the system faultless by correcting these errors. Consequently, the FEC function can provide a dynamic evaluation of the system margins relative to the required level of performance. There are various FEC codes, e.g., the (255,239) Reed-Solomon code (ITU G.975), well known in the art, has been adopted by the ITU for submarine fiber systems.
Solheim et al. in U.S. Pat. No. 5,896,391 xe2x80x9cForward Error Correction Assisted Receiver Optimizationxe2x80x9d issued Apr. 20, 1999, describe a method of setting the eye opening phase and decision level to optimize the BER of a digital communication network. The BER of a digital receiver depends on the decision threshold and phase. For a symmetric eye, and equal noise probabilities on zeros and ones, the ideal decision threshold is the average power level, at the midpoint between the transition points. If the pulses are distorted, or if there is signal-dependent noise, then the optimum decision threshold point may be displaced from the midpoint. The optimum decision threshold and phase of a receiver can be found by sweeping the decision level and phase, in sequence, while monitoring the line BER, then setting the phase and decision level at the point that optimized the FEC.
FEC is typically used to provide significant savings in the overall optical power budget of a fiber link, by allowing the system to operate at a much higher line BER. For example, the (255,239) Reed-Solomon code (cited above) corrects a 10xe2x88x924 line BER to 5*10xe2x88x9215. Thus, with FEC the communication link can support line BER rates up to about 10xe2x88x924, report on the high BER in real-time, and correct the errors to better than 10xe2x88x9212 (a typical target maximum BER for optical communication networks).
Despite the technical advances described above, there is no broadly applicable method for automatic optimization of digital communication networks. There is a need, therefore, for a method of optimizing the performance of such networks. Further, there is a need for such a method that works automatically and is broadly applicable with respect to variable network parameters. Moreover, there is a need for such a method that allows for in-service optimization of both single-channel and multi-channel digital communication networks.
Accordingly, it is a primary object of the present invention to provide a method of optimizing the performance of digital communication networks. Further, it is an object of the invention to provide such a method that works automatically and is broadly applicable with respect to variable network parameters. Moreover, it is an object of the invention to provide such a method that allows for in-service optimization of both single-channel and multi-channel digital communication networks.
In accordance with embodiments of the invention, a method of optimizing the performance of digital communication networks is described. The method measures the line bit error rate (BER) at a receiver by counting corrected errors, as reported by a forward error correction (FEC) decoder, and adjusts network parameters to minimize the corrected error rate. The method can be applied to any digital communication network, and has particular applicability for fiber optic communication networks. Additionally, the method can be applied to multi-channel networks, including fiber optic networks having multiple transmitters, receivers, and variable parameters, to balance the performances of different channels. The method can also minimize interchannel crosstalk, by optimizing the corrected errors as multiplexing parameters are adjusted. The method can for example optimize the signal wavelengths in a wavelength division multiplexed (WDM) network.
Embodiments of the present invention provide methods for in-service optimization of various adjustable parameters in an optical communication network. Adjustable parameters include, among other things, transmitter wavelength, transmitter power, transmitter extinction ratio, transmitter polarization, and dispersion of a dispersion compensator. The methods include sweeping an adjustable parameter while monitoring the FEC corrected bit error rates at the receiver(s). By measuring the line BER versus the value of the adjustable parameter, the optimum value of a given parameter can be extrapolated. Since the FEC corrects for the line errors, as long as the line BER is low enough, no bit errors will be produced. Accordingly, a parameter can be adjusted while the network is in-service, for example at regular maintenance intervals or at an upgrade of the network. If the BER exceeds a level such that it is no longer correctable by the FEC, e.g. greater than 10xe2x88x923, e.g., when the digital communication network is installed, then the parameter adjustment is performed out-of-service.
In an embodiment of the present invention, a variable parameter is adjusted by increasing the parameter from a predetermined nominal operating value until the corrected FEC errors at the receiver reach a predetermined BER limit. Then the parameter is decreased from the nominal operating value until the corrected FEC errors again reach the predetermined BER limit. In this way, the corrected BER is maintained below the predetermined BER limit while the digital communication network is in-service.
In a multi-channel digital communication network, for example a multi-channel fiber optic network sharing a common optical amplifier among multiple channels, the channels are balanced by adjusting a plurality of variable parameters, while monitoring the corrected FEC errors at each receiver. The value of each parameter at which each respective receiver reaches a predetermined BER limit is determined, and the values of the respective parameters are extrapolated, such that each transmitter-receiver channel has an equal relative power difference above the BER limit.
When a new channel is added to the network, its performance is balanced with the existing channels by decreasing the transmitter power of the new channel until the BER at the new channel receiver reaches the predetermined BER limit of the existing network channels. Then the transmitter power of the new channel is set such that the relative transmitter power difference from the power at the BER limit of the new channel equals the relative transmitter power difference from the respective transmitter powers at the respective BER limits of the existing channels.